you don't qualify under the immigration rules to switch into a spouse category, because as others have correct posted, your wife is not settled in the UK (she only has limited leave not ILR) however this is a developing area in terms of case law and the courts have stated that the immigration rules should be changed to allow those with limited leave as refugees to be able to sponsor dependents as others with limited leave are.
The cases in particular are:
A (Afghanistan) v SSHD [2009] EWCA Civ 825 - you'll find this if you google it
and - FH (Post-flight spouses) Iran [2010] UKUT 275 (IAC). - follow the link below
http://www.bailii.org/uk/cases/UKUT/IAC ... _iran.html
I would suggest that you can make an application arguing that you meet all of the requirements of pargraph 284 of the immigration rules (including accomodation and maintenance) but for the fact that your wife does not have settled status and make reference to these two cases.
You can make the application on form FLR(O) or FLR(M). However would recommend that you seek advice from an immigration solicitor/advisor on making the application.